A power output of an electric power generator based on renewable energy such as sunlight or wind is still unstable. In addition, it is known that its output state remarkably varies depending on an environmental condition. Therefore, it is desirable to stabilize the output power in the future.
Since such a renewable-energy-based electric power generator is regarded as being unsuitable for use as a stable power source, accumulation in a rechargeable battery was proposed. However, in this case, a power loss is generated in a charging/discharging operation disadvantageously.
In the renewable-energy-based electric power generator (such as a solar cell panel or a wind turbine), the output power varies depending on a current-voltage condition at the time of power output operation. Therefore, it is indispensable to perform control for outputting the electric power under an optimum condition.
For example, if the output is short-circuited while a solar cell panel is laid under direct sunlight, the output current is maximized to 5.29 A (refer to a point A in FIG. 3). However, the voltage becomes zero (0 V). Therefore, the output power becomes zero (0 W).
In contrast, if the output is opened, the output voltage is maximized to 22.59 V (refer to a point B in FIG. 3). However, the current becomes zero (0 A). Similarly, in this case, the output power becomes zero (0 W)
When the output voltage is at 18.14 V, the output current becomes 4.97 A (refer to a point P in FIG. 2). In this case, a maximum power of, approximately, 90 W can be output (maximum power point). Even in a lower or higher voltage, the output power is reduced. In order to output the maximum power, for example, a so-called maximum power point tracking (MPPT) technique has been applied to a lot of power conditioners (refer to FIG. 4).
Typically, it is known that, in an apparatus for combining a plurality of DC power sources of the related art, a so-called reverse current prevention circuit (a circuit for preventing a reverse current flow, such as a reverse current prevention diode) or a DC converter (such as a PWM chopper circuit) used to supply power to a DC load with a higher priority from a DC power source side may be installed solely or in combination.
In this regard, in the case of combining a plurality of power sources, when the power sources have different characteristics (such as an open circuit voltage), the current may flow between a plurality of power sources, and this may generate a failure.
For example, if the reverse current prevention circuit is not provided in the DC converters 30 and 31 as illustrated in FIG. 8, the output voltages V1 and V2 of the DC converters 30 and 31 have a relationship of “V1>V2,” and a load resistance is high (for example, if no load is connected), the current may flow to the DC converter 31 side as indicated by the arrow 32, and this may generate a failure (refer to FIG. 8).
In order to prevent such a failure, the reverse current is blocked by providing reverse current prevention circuits 33 (for example, reverse current prevention diodes) as illustrated in FIG. 9.
Inevitably, an electric power loss is generated by providing the reverse current prevention circuit 33 described above when the electric current flows through the reverse current prevention circuit 33. Assuming that a reverse current prevention diode is employed in the reverse current prevention circuit 33, for example, an electric power loss of 0.7 W may be generated for an electric current of 1 A due to a forward bias voltage of, approximately, 0.7 V in a typical diode.